The present invention relates to methods and compositions for controlling the migration of particulates, (e.g., proppant and formation fines) within a portion of a subterranean formation. More particularly, the present invention relates to remedial and proactive methods (relative to a fracturing treatment) for controlling the migration of particulates in subterranean formations.
Hydraulic fracturing is a process commonly used to increase the flow of desirable fluids from subterranean formations, such as coal formations, carbonate formations, sandstone formations, shaly formations, and mixtures thereof. Traditional hydraulic fracturing operations comprise placing a viscous fracturing fluid into a portion of a subterranean formation at a rate and pressure such that fractures are formed or enhanced into the portion of the subterranean formation. The fractures tend to propagate as vertical and/or horizontal cracks located radially outward from the well bore. In such treatments, once the hydraulic pressure is released, the fractures formed will tend to close back onto themselves. To prevent this, oftentimes a particulate material, known as proppant, is placed in the fractures by suspending them in the fracturing fluid during at least a portion of the fracturing operation. The particulates are carried into created fractures and deposited therein such that when the hydraulic pressure is released the particulates act to prevent the fracture from fully closing, and thus, aid in forming conductive channels through which produced fluids may flow into the well bore. The term “propped fracture” as used herein refers to a fracture in a portion of subterranean formation that contains some proppant particulates. The term “proppant pack” refers to a collection of a mass of proppant particulates within a fracture. Without the particulate material, the fractures tend to close and reduce permeability gained by the fracturing operation. Suitable particulate materials should have sufficient compressive strength to resist crushing, but also must be sufficiently non-abrasive and non-angular to preclude cutting and imbedding into the formation.
Hydrocarbon wells are often located in subterranean zones that contain unconsolidated particulates (e.g., proppant and formation fines) that may migrate within the subterranean formation with the oil, gas, water, and/or other fluids produced by a well penetrating the subterranean formation. The presence of these unconsolidated particulates in produced fluids is disadvantageous and undesirable in that the particulates may abrade pumping and other producing equipment and reduce the fluid production capabilities of producing zones. Unconsolidated subterranean zones include those that contain loose particulates and those wherein the bonded particulates have insufficient bond strength to withstand the forces produced by the production of fluids through the zones. “Zone” as used herein simply refers to a portion of the formation and does not imply a particular geological strata or composition.
One traditional method of controlling unconsolidated particulates in zones of a subterranean formation involves placing a filtration bed containing gravel particulates near the well bore that neighbors the zone of interest. The filtration bed acts as a sort of physical barrier to the transport of unconsolidated particulates to the well bore that could be produced with the produced fluids. Typically, such so-called “gravel packing operations” involve the pumping and placement of a quantity of desired particulates into the unconsolidated formation in an area adjacent the well bore. One common type of gravel packing operation involves placing a sand control screen in the well bore and packing the annulus between the screen and the well bore with gravel of a specific size designed to prevent the passage of formation sand. The sand control screen is generally a filter assembly used to retain the gravel placed during gravel pack operation. A wide range of sizes and screen configurations are available to suit the characteristics of the gravel pack sand used. Similarly, a wide range of sizes of gravel is available to suit the characteristics of the unconsolidated particulates. The resulting structure presents a barrier to migrating sand from the formation while still permitting fluid flow. When installing the gravel pack, the gravel is carried to the annulus in the form of a slurry by mixing the gravel with a viscous fluid, often known as a “gravel pack fluid.” Once the gravel is placed in the well bore, the viscosity of the fluid is reduced, and it is returned to the surface. In some gravel packing operations, commonly known as “high rate water packing operations,” the viscous fluid has a lower viscosity and yet the gravel remains in suspension because the treatment occurs at a high velocity. Gravel packs act, inter alia, to stabilize the formation while causing minimal impairment to well productivity. The gravel, inter alia, acts to prevent the particulates from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the production tubing. Such packs may be time consuming and expensive to install.
Another method used to control particulates in unconsolidated formations involves consolidating unconsolidated portions of subterranean producing zones into relatively hard permeable masses by applying a resin followed by a spacer fluid and then a catalyst. Such methods may be problematic when, for example, an insufficient amount of spacer fluid is used between the application of the resin and the application of the external catalyst. In that case, the resin may come into contact with the external catalyst in the well bore itself rather than in the unconsolidated subterranean producing zone. Furthermore, there is uncertainty as to whether there is adequate contact between the resin and the catalyst. Additionally, when resin is contacted with an external catalyst an exothermic reaction occurs that may result in rapid polymerization, potentially damaging the formation by plugging the pore channels, halting pumping when the well bore is plugged with solid material, or resulting in a down hole explosion as a result of the heat of polymerization. Uniform placement of curable resin into the formations having long intervals is most desirable. However, formations often comprise a wide range of permeabilities even within a reservoir located along a well bore. As a result, completions involving resin consolidation, with conventional diversion techniques, have been applied in intervals of less than 50 feet, and more ideally, less than 30 feet. Also, using resins to consolidate long or large unconsolidated zones may not be practical due, at least in part, to the high cost of most suitable resins.
Another similar method involves applying a non-aqueous tackifying composition to the unconsolidated particulates in an effort to reduce the migration of particulates within the zone. Whereas a curable resin composition produces a hard mass, the use of a non-aqueous tackifying composition produces a more malleable consolidated mass.
Another alternative is an aqueous tackifying composition. Aqueous tackifying compositions, however, have their own problems including, but not limited to, the fact that they require external activators and surfactants for optimum performance.